A variety of different topologies have been developed and employed to provide isolated AC-to-DC and DC-to-DC power conversion. A sub-group of these converter topologies, sometimes called Current Fed topologies are in many respects advantageous over another sub-group, the so-called Voltage Fed converter topologies. These advantages relate to a potential circuit simplicity and its smaller component count. Indeed, a single stage PFC (power factor corrected) isolated AC-to-DC converter (realized in Current Fed topology) is often more cost effective than a traditional two stage solution. However, in order to realize these advantages, it is necessary to overcome the inherent problem in the Current Fed topologies--containment and proper disposal of energy stored in the leakage and magnetizing inductances of the isolation transformer.
Many methods were developed to accomplish this task with a varying success. The drawbacks of these circuits are: uneven stress distribution between the switches; DC current bias in the power transformer; and, necessity of employing some damping (snubbing) circuits. These drawbacks limit the use of these circuits to relatively low power.
The Push-Pull Boost current fed topology has been widely described. Its simplicity is negated by such drawbacks as high voltage stress on the switches, and necessity of employing damping (snubbing) circuits to cope with voltage spikes caused by the unclamped leakage inductance of the transformer.
Another example of the full bridge topology utilization are Current Fed Full-Bridge converters which utilize some ancillary circuitry to facilitate zero voltage transition (ZVT) operation of the bridge switches. In these converters, the ancillary circuit is placed in the secondary. This ancillary circuit is comprised of two MOSFETs, resonant inductor and capacitor which provide ZVT conditions for the primary bridge switches, thus enhancing their switching efficiency. However, the benefits of it are negated by the necessity of using inefficient bridge rectifier circuit and by the four series/parallel secondary connected MOSFETs. The circuit overall is complex and component extensive (expensive).
The common drawback of the above mentioned circuits is an inadequate provision for the transformer magnetizing current flow and the necessity of additional damping or snubbing circuitry to cope with voltage spikes caused by the unclamped transformer leakage inductance.
An active clamp circuit comprised of the capacitor in series with the MOSFET has been proposed for use with a variety of converter circuits in the past. The capacitor/transistor active clamp circuit has been applied to a single ended forward converter and to DC-to-DC non-isolated boost converters. However, these do not consider the Isolated Current Fed Full-Bridge which operates in a very specific manner.
An Isolated Current Fed converter topology is needed that provides for transformer magnetizing current flow, and does not require additional damping, or snubber circuitry, to cope with voltage spikes caused by unclamped transformer leakage inductance. In addition, such converter should exhibit soft switching operation for high efficiency and low noise.